Various multi plate heat exchangers have been devised prior to the present invention to increase their heat exchanger efficiency and usefulness. Among these heat exchangers is the type disclosed in my copending application U.S. Ser. No. 677,193 filed Mar. 29, 1991 for HIGH EFFICIENCY HEAT EXCHANGER WITH DIVIDER RIB LEAK PATHS, assigned to the assignee of this invention and hereby incorporated by reference. In my copending application, spaced leak paths were provided in the centralized divider rib of U-flow type evaporator tubes to ensure that some of the refrigerant would be supplied to the outlet or vapor side of the tube so that localized dry out and hot spots would be reduced or eliminated and heat exchanger efficiency would be thereby improved.
This heat exchanger is of the general category of that disclosed in my co-pending application but has a plurality of flattened tubes which are operatively joined at upper and lower tank ends to form a core for the passage of volatile heat exchanger fluid therethrough from an intake pipe to an outlet. Each of these tubes are formed from a pair of plates having indented rib patterns therein to vary the flow path through the tube to enhance the heat exchanger efficiency.
This invention is preferably employed as tubing for an evaporator for an air conditioner in which the heat exchanger fluid changes state from a liquid phase to a gaseous phase as it courses through the evaporator tubing. Such heat exchanger tubes are generally oval or rectilinear in shape, and are arranged into a core so that the tubes have thin leading and trailing edges with sides flattened to provide large area heat exchanging surfaces therebetween. Air centers are conventionally installed between the walls defining the sides of the tubes to increase the efficiency of the core. This construction also allows the air to flow across the large surface areas of the tubes with minimized resistance.
With prior constructions local dry out areas may occur near the outlet ends of each tube causing the heat transfer rate to be significantly reduced. This reduction occurs because the gaseous phase of the fluid has lower heat transfer efficiency than the liquid phase or when the fluid in a transition liquid and gas phase. Furthermore, the front or air inlet side of the heat exchanger is more effective in absorbing heat transfer than the back or air outlet side thereof. This is caused in part by the fact that the front side of the heat exchanger encounters and absorbs more heat energy than the back side as air is blown past the tubes of the core. Accordingly, dry out or hot spot areas generally occur near the heat exchanger outlet of each tube and near the front or air inlet side.
In view of the above, this invention provides a new and improved evaporator which features unique construction that eliminates or sharply reduces local dry out areas in an evaporator by improving control of the change in phase from a liquid to a gas as the heat exchanger fluid courses through the heat exchanger tubing from the inlet to the outlet thereof. More particularly, by feeding increased quantities of heat exchanger liquid or by feeding a mixture containing higher quantity of liquid than vapor to the lower or outlet section of each tube, dry out areas will be significantly reduced and heat transfer efficiency will be improved.
Accordingly, it is a feature, object and advantage of this invention to provide a new and improved tube for use in a heat exchanger core in which heat exchanger fluid flow paths are provided from the heat exchanger inlet of the tubes to the outlet thereof so that increased quantities of volatile liquid can be fed to the outlet or lower sections of the tube so as to be available for vaporization in otherwise dry out areas to thereby increase the heat transfer efficiency of the heat exchanger tubing.
In a preferred embodiment of the present invention, dry out can be effectively eliminated by providing a rib pattern which enhances heat exchanger fluid flow in discrete sections of the tubing. These patterns are arranged to keep the lower part of each tube adequately fed with liquid heat exchanger fluid so that all portions of the tubing are effectively used to absorb the heat energy of the air blowing past the tubes to change the phase of the heat exchanger fluid into a gas phase.
The tube pass of this invention provides a highly efficient heat transfer design by providing a first tube section with a plurality of transversely extending rows of ribs having a crossed rib configuration in a first zone to provide a tortuous flow path for fluid for high efficiency heat transfer operation. The tube pass has a second and downstream zone or section with many avenues of flow including free bypass flow around the aligned rib rows so that heat exchanger liquid can pass therethrough without substantial vaporization, i.e., reduced efficiency. This tube pass further provides a third zone with an overlapping rib configuration which receives sufficient liquid from the second zone so that the heat absorption and efficiency in a third zone is enhanced since dry out areas are reduced or eliminated. Moreover, this arrangement provides excellent fluid distribution across the width of the tube pass and within the tube for efficient use of the extensive heat transfer area thus provided.